1. Technical Field
The present invention relates generally to a filter and filter element construction. In particular, the present invention relates to a filter element that includes a supporting substrate made from at least two materials with different melt temperatures and to a method of making the filter clement.
2. Description of the Prior Art
It is known that efforts throughout the world to improve air quality continually increase through regulations which limit the amount of allowable industrial emissions released into the atmosphere. Improved and more efficient filters that minimize and reduce the release of emissions have been developed. One such filter includes a filter element with a microporous expanded polytetrafluoroethylene (ePTFE) membrane. The membrane assumes a relatively large percentage of the filtering duties when attached to a support structure, such as a substrate.
It is known that problems can result from attaching together the membrane and the substrate. For example, the membrane may be attached to the substrate by a laminating operation that typically uses a thermal bonding process. In this thermal bonding process, fibers at an external surface of the substrate are singed to increase the area of the substrate that can contact the membrane. The membrane and substrate are brought into intimate contact. The singed areas of the substrate are then melted onto the membrane by the application of heat. The melted portions of the substrate flow into pores in the membrane. Once the melted portions of the substrate solidify, the membrane is fixed to the substrate.
However, the material used in the substrate can melt uncontrollably and engage a relatively large surface area of the membrane. Melted portions of the substrate that solidify can block a substantial portion of the pores in the membrane and reduce the effective filtering area of the membrane. Thus, filtration efficiency is detrimentally affected due to the inhibited airflow and a relatively large pressure drop across the filter element that can result. It is also known that as melted portions of the substrate solidify to attach to the membrane, the melted portions tend to shrink in size. This shrinkage can overstress the membrane and cause the membrane to tear or crack. This tearing or cracking of the membrane may also occur when the filter element is thermally cycled in use.
Therefore, there is a need in the filtration art for an improved filter element that is durable in various environments.